Primary production and its subsequent loss by consumption and physical export regulate the turnover rate of phytoplankton biomass. Variations in turnover rates have important implications for the functional composition of pelagic communities, the trophic structuring of biomass and the limits to nutrient cycling and primary production. The extent to which turnover rates remain constant or vary systematically across broad ranges of aquatic ecosystems is unclear. We have synthesized a large database on reported turnover rates of pelagic autotrophic biomass across broad gradients of productivity and nutrient availability in freshwater (n=401) and coastal marine systems (n=121). Across systems, turnover rates showed no systematic tendency to increase or decrease along gradients of productivity and nutrient availability. In addition, our results indicate that turnover rates in coastal marine systems are nearly 3-fold higher than those in freshwater systems. The cross-system patterns of turnover rates reported here point to fundamental differences in the structure and fates of primary production between marine and freshwater planktonic systems.